Field of the Invention
The present invention relates to a control apparatus of an internal combustion engine, and more particularly, to a control apparatus of an internal combustion engine configured to calculate an amount of air sucked into cylinders with accuracy.
Description of the Related Art
In order to control an internal combustion engine suitably, it is crucial to perform fuel control and ignition control according to an amount of air sucked into cylinders by calculating the amount of air sucked into the cylinders with a high degree of accuracy. As a method of measuring an amount of air sucked into the cylinders of the internal combustion engine, a pressure sensor (hereinafter, referred to as the intake manifold pressure sensor) is provided to measure a pressure in a portion including a surge tank and an intake manifold (hereinafter, collectively referred to as the intake manifold) located downstream of a throttle valve, and an amount of air sucked into the cylinders is calculated on the basis of an intake manifold pressure measured by the intake manifold pressure sensor and a rotational speed of the internal combustion engine (hereinafter, referred to as the engine speed) (this method is known as the Speed Density method and hereinafter referred to as the S/D method). Because the intake manifold pressure sensor is relatively inexpensive, the S/D method is employed extensively.
An example of the S/D method is disclosed, for example, in JP-A-08-303293 (Patent Document 1). Patent Document 1 discloses that an amount of cylinder suction air is calculated on the basis of an intake manifold pressure, a volumetric efficiency equivalent value (referred to as the volumetric efficiency VE in Patent Document 1), which is an index of an amount of air sucked from the intake manifold into the cylinders, a cylinder volume V, and a temperature T. The volumetric efficiency VE is stored in a single map with axes representing the intake manifold pressure and the engine speed.
In order to achieve further lower fuel consumption and a higher output, an engine that is becoming popular in recent years is equipped with a VVT (Variable Valve Timing) mechanism (hereinafter, referred to as the intake VVT) that allows opening and closing timing of an intake valve to vary. In the engine equipped with the intake VVT, an amount of an exhaust gas flown back from an exhaust path to the cylinders and an actual compression ratio vary with a difference of the valve opening and closing timing. Accordingly, even under the same conditions of the intake manifold pressure and the engine speed, an amount of cylinder suction air varies considerably depending on a difference of the valve opening and closing timing. Hence, unless influences of the valve opening and closing timing on the volumetric efficiency VE are taken into consideration, a degree of calculation accuracy of an amount of cylinder suction air is lowered over the entire steady and transient operating ranges in the S/D method disclosed in Patent Document 1.
To overcome the problem as above, there is a method taking an engine equipped with the intake VVT into consideration as is described, for example, in JP-A-2008-138630 (Patent Document 2). Patent Document 2 discloses a method (AFS method) by which an amount of air is measured by an AFS (Air Flow Sensor) provided upstream of a throttle valve of an intake pipe of the engine. An intake system is modeled according to the mass conservation law alone and an amount of cylinder suction air is calculated using a volumetric efficiency correction coefficient. The technique described in Patent Document 1 and the technique described in Patent Document 2 consider so-called a state equation of ideal gas (P=ρRT, where P is a pressure, ρ is a density, R is a gas constant, and T is a temperature). It can be assumed that a volumetric efficiency correction coefficient in Patent Document 2 and the volumetric efficiency VE in Patent Document 1 are equivalent (hereinafter, these terms as well as the one used herein are referred to as the volumetric efficiency equivalent value Kv). In the engine unequipped with the intake VVT, the volumetric efficiency equivalent value Kv is stored in a single map with axes representing the intake manifold pressure and the engine speed as is in Patent Document 1.
In the method disclosed in Patent Document 2, one volumetric efficiency equivalent value Kv map is held for every operating condition of an intake VVT phase angle. For example, in a case where an operating range of the intake VVT phase angle is indicated by six representative points and intervals are interpolated, six volumetric efficiency equivalent value Kv maps are held. When configured in this manner, it becomes possible to calculate an amount of cylinder suction air in consideration of influences of the valve opening and closing timing on the volumetric efficiency equivalent value Kv.
In the method of Patent Document 1, it is also possible to calculate an amount of cylinder suction air in consideration of influences of the valve opening and closing timing on the volumetric efficiency equivalent value Kv by holding the volumetric efficiency equivalent value Kv map for every operating condition of the intake VVT phase angle.
Patent Document 1: JP-A-08-303293
Patent Document 2: JP-A-2008-138630
Incidentally, there is a turbocharger having a supercharger that is installed in an intake path of the engine and driven by rotating a turbine with an exhaust gas. The turbocharger generally has an exhaust bypass passage located upstream of the turbine. The turbocharger regulates an amount of an exhaust gas flown into the turbine by diverting a part of the exhaust gas flown through an exhaust path into the bypass passage using a waste gate valve (hereinafter, referred to as the W/G valve) provided to the exhaust bypass passage, and controls a supercharging pressure to be maintained at an adequate level.
More specifically, in a case where an opening degree of the W/G valve is controlled to be on an opening side, an amount of the exhaust gas flown into the turbine is decreased and a supercharging pressure drops, whereas in a case where an opening degree of the W/G valve is controlled to be on a closing side, an amount of the exhaust gas flown into the turbine is increased and a supercharging pressure rises. In this instance, a pressure in the exhaust path located upstream of the turbine (hereinafter, referred to as the exhaust pressure) varies, too. Hence, in a case where an opening degree of the W/G valve is controlled to be on the opening side, the exhaust pressure drops whereas the exhaust pressure rises in a case where an opening degree of the W/G valve is controlled to be on the closing side.
Under the same conditions of the intake manifold pressure, the engine speed, and the valve opening and closing timing, an amount of the exhaust gas flown back from the exhaust path to the cylinders increases when the exhaust pressure is high, whereas an amount of the exhaust gas flown back from the exhaust path to the cylinders degreases when the exhaust pressure is low. In other words, in the turbocharged engine in which the exhaust pressure varies considerably with an opening degree of the W/G valve even under the same conditions of the intake manifold pressure, the engine speed, and the valve opening and closing timing, a degree of calculation accuracy of an amount of cylinder suction air becomes poor unless consideration is given to influences of the exhaust pressure on the volumetric efficiency equivalent value Kv.
Hereinafter, influences of the exhaust pressure on the volumetric efficiency equivalent value Kv will be described in detail. A relation of an amount of cylinder suction air and the volumetric efficiency equivalent value Kv is expressed by Equation (1) as follows:[Mathematical Formula 1]Q=Kv×Pb×Vc÷(Tb×R×T_SGT)  (1)where Q is an amount of cylinder suction air [g/s], Kv is the volumetric efficiency equivalent value, Pb is an intake manifold pressure [kPa], Vc is a cylinder displacement [L], Tb is an intake manifold temperature [K], R is a gas constant [J/(kg·K)], and T_SGT is a predetermined crank angle interval [sec] (in the case of a four-cylinder engine, the interval is 180 degrees and in the case of a three-cylinder engine, the interval is 240 degrees).
Equation (1) above is the same as the one used in Patent Document 1. In accordance with Equation (1), the volumetric efficiency equivalent value Kv can be calculated using an amount of cylinder suction air, Q (g/s), an intake manifold pressure Pb (kPa), a cylinder displacement Vc (L), an intake manifold temperature Tb (K), a gas constant R (J/kg·K), and a predetermined crank angle interval T_SGT (sec). The volumetric efficiency equivalent value Kv in each operating range of an engine of interest is obtained by a simulation in actual use. By storing the obtained volumetric efficiency equivalent values Kv in the volumetric efficiency equivalent value Kv map with axes representing an intake manifold pressure and an engine speed, a volumetric efficiency equivalent value Kv is calculated during the actual engine control using an intake manifold pressure, an engine speed, and the volumetric efficiency equivalent value Kv map.
Images I through III of FIG. 14 are image views showing relations of an exhaust pressure, an internal EGR ratio {=partial pressure of burned gas in cylinders when the intake value is closed÷(partial pressure of burned gas in cylinders when the intake value is closed+partial pressure of air in cylinders when the intake valve is closed)}, and an amount of cylinder suction air with respect to an intake manifold pressure at the same engine speed. In each image view, a solid line indicates a relation when the W/G valve is fully closed (on the supercharging side) and an alternate long and short dash line indicates a relation when the W/G valve is fully opened (on the relief side). It should be noted that the valve opening and closing timing of the intake valve is the same when the W/G valve is fully closed and fully opened.
A relation of an exhaust pressure with respect to an intake manifold pressure will be described using Image I of FIG. 14. In Image I, the abscissa is used for the intake manifold pressure and the ordinate is used for the exhaust pressure.
In a region where the intake manifold pressure is lower than the one on a vertical broken line A, the exhaust pressure is substantially equal when the W/G valve is fully opened and fully closed. The reason underlying this result is that an amount of an exhaust gas flown into the turbine is too small for the turbine to rotate high enough for supercharging (the exhaust pressure does not rise, either) regardless of whether the W/G valve is fully opened or fully closed. In a region where the intake manifold pressure is in a range from the one on the vertical line A to the one on a vertical line B, an amount of an exhaust gas flown into the turbine is large and the turbine rotates high enough for supercharging when the W/G value is fully closed. At the same time, because resistance when the exhaust gas passes through the turbine increases, the exhaust pressure rises above atmospheric pressure. On the other hand, when the W/G valve is fully opened, because most of the exhaust gas passes through the exhaust bypass passage side, the exhaust pressure exceeds atmospheric pressure only slightly. In a region where the intake manifold pressure is higher than the one on the vertical broken line B, a flow rate of the exhaust gas is so high that an exhaust gas cannot be released sufficiently toward the exhaust bypass passage even when the W/G valve is fully opened. Hence, because an amount of the exhaust gas flown into the turbine increases, the exhaust pressure rises above atmospheric pressure.
A relation of the internal EGR ratio with respect to an intake manifold pressure will now be described using Image II of FIG. 14. In Image II, the abscissa is used for the intake manifold pressure and the ordinate is used for the internal EGR ratio.
In a region where the intake manifold pressure is lower than the one on the vertical broken line A, the internal EGR ratio with respect to the same intake manifold pressure is substantially equal when the W/G valve is fully opened and fully closed. The reason underlying this result is that because the exhaust pressure is substantially equal when the W/G valve is fully opened and fully closed (see Image I), an amount of the exhaust gas flown back from the exhaust path to the cylinders is also substantially equal when the W/G valve is fully opened and fully closed. In a region where the intake manifold pressure is in a range from the one on the broken line A to the one on the broken line B, the internal EGR ratio with respect to the same intake manifold pressure is lower when the W/G valve is fully opened than when the W/G valve is fully closed. The reason underlying this result is that because the exhaust pressure with respect to the same intake manifold pressure is lower when the W/G valve is fully opened than when the W/G valve is fully closed (see Image I), an amount of the exhaust gas flown back from the exhaust path to the cylinders decreases when the W/G valve is fully opened in comparison with an amount of the flown back exhaust gas when the W/G valve is fully closed. In a region where the intake manifold pressure is higher the one on the vertical broken line B, a difference between the internal EGR ratios with respect to the same intake manifold pressure when the W/G valve is fully opened and fully closed becomes smaller as the intake manifold pressure rises. The reason underlying this result is that because an amount of cylinder suction air increases as the intake manifold pressure rises, a space in the cylinders for the exhaust gas flown back from the exhaust path to the cylinders becomes smaller, and therefore a difference between amounts of the exhaust gas flown back from the exhaust path to the cylinders with respect to the same intake manifold pressure becomes smaller even when there is a difference between the exhaust pressures when the W/G valve is fully opened and fully closed (see Image I).
A relation of an amount of cylinder suction air with respect to an intake manifold pressure will now be described using image III of FIG. 14. In Image III, the abscissa is used for the intake manifold pressure and the ordinate is used for an amount of cylinder suction air, Q.
In a region where the intake manifold pressure is lower than the one on the vertical broken line A, because the internal EGR ratio with respect to the same intake manifold pressure is substantially equal when the W/G valve is fully opened and fully closed (see Image II), an amount of cylinder suction air, Q, with respect to the same intake manifold pressure is substantially equal when the W/G valve is fully opened and fully closed. In a region where the intake manifold pressure is in a range from the one on the vertical broken line A to the one on the vertical broken line B, the internal EGR ratio with respect to the same intake manifold pressure is lower when the W/G valve is fully opened than when the W/G valve is fully closed (see Image II). Hence, an amount of cylinder suction air, Q, with respect to the same intake manifold pressure increases when the W/G valve is fully opened in comparison with an amount of cylinder suction air, Q, when the W/G valve is fully closed. In a region where the intake manifold pressure is higher than the one on the vertical broken line B, a difference between the internal EGR ratios with respect to the same intake manifold pressure when the W/G valve is fully opened and fully closed becomes smaller as the intake manifold pressure rises. Hence, a difference between amounts of cylinder suction air, Q, with respect to the same intake manifold pressure when the W/G valve is fully opened and fully closed becomes smaller.
As has been described, even under the same conditions of an intake manifold pressure and an engine speed, an amount of cylinder suction air varies considerably depending on a difference of the exhaust pressure. In a case where no consideration is given to influences of the exhaust pressure on the volumetric efficiency equivalent value Kv calculated during the actual engine control using an intake manifold pressure, an engine speed, and the volumetric efficiency equivalent value Kv map, there arises a problem that a degree of calculation accuracy of an amount of cylinder suction air becomes lower. For example, assume a case where the volumetric efficiency equivalent value Kv when the W/G valve is fully closed is calculated in accordance with Equation (1) above and pre-stored in a map with axes representing an intake manifold pressure and an engine speed for use during actual engine control. When the W/G valve is controlled to be fully closed, an amount of cylinder suction air, Q, calculated in an engine control computer unit (hereinafter, referred to as the ECU) is calculated with accuracy for an actual amount of air. There is, however, a case where an amount of cylinder suction air, Q, calculated in the ECU is smaller than the actual amount of air when the W/G valve is controlled to be on the opening side and the exhaust pressure drops from the one when the W/G valve is fully closed at the same intake manifold pressure and engine speed.
Regarding the problem above, consideration of influences of the exhaust pressure on the volumetric efficiency equivalent value Kv is described in neither Patent Document 1 nor Patent Document 2.
In addition, as in the case where consideration is given to influences of the valve opening and closing timing on the volumetric efficiency equivalent value Kv, the volumetric efficiency equivalent value Kv map may be held for every operating condition of the W/G valve. In a case where an operating range of the W/G valve is indicated by six representative points and intervals are interpolated, six volumetric efficiency equivalent value Kv maps are held. It is possible to consider influences of the exhaust pressure on the volumetric efficiency equivalent value Kv by this method. However, in an engine equipped with the intake VVT and the turbocharger, six volumetric efficiency equivalent value Kv maps for consideration of the valve opening and closing timing are held for each of the six representative points of the operating range of the W/G valve. In short, 6×6, that is, 36 volumetric efficiency equivalent value Kv maps are necessary. Hence, there are problems that a large number of man hours are required for adaptation and data setting and that a microcomputer in the ECU requires a huge memory capacity.
In a case where an opening degree of the W/G valve is controlled in such a manner that the W/G valve opens at a unique opening degree with respect to an intake manifold pressure and an engine speed, the exhaust pressure also takes a unique value with respect to an intake manifold pressure and an engine speed. Hence, no consideration is necessary for a difference of the exhaust pressures at the same intake manifold pressure and engine speed. In this case, however, there is a problem that it becomes impossible to control the W/G valve at an arbitrary opening degree (for example, acceleration cannot be achieved by raising a supercharging pressure by controlling the W/G valve to be on the closing side temporarily in response to an acceleration request from the driver).